Storage units for electrical energy are usually constructed from individual storage elements. The nominal operating voltage of these storage elements is usually relatively low, e.g. in the range of between 2 and 2.5 V with double-layer capacitors. For most applications, however, a far higher operating voltage of the energy storage unit is required. For example, the electric drives of hybrid motor vehicles are operated at between 48 and 300 V, depending on the type, so that an energy storage unit designed to feed a drive of this type needs to have a correspondingly high operating voltage. In order to attain this high voltage, with energy storage units, a corresponding number of storage elements are typically switched in series. For example, a 48 V energy storage unit can be created by a series connection of 20 double-layer capacitors with an operating voltage of approx. 2.4 V.
It is now known that the working life of energy storage units of this type is significantly reduced by a lack of homogeneity in the charge state of the individual storage elements (for example from H. Schmidt et al. “The charge equalizer—a new system to extend battery lifetime in photovoltaic systems, U.P.S. and electric vehicles”, International Telecommunications Energy Conference, Intelec, Paris, 27 to 30 Sep. 1993, IEEE vol. 2, Conf. 15, p. 146-151). Previously, it was assumed that with a series connection of individual storage elements, all these elements would have identical properties, and would constantly be in the same charge state. In actual fact, the storage elements—which are nominally the same—generally deviate slightly from each other in terms of their nominal values (such as capacity and self-discharging rate). With a simple series connection, differences of this nature can on the one hand, during discharging processes, lead to total discharges or even to inverse charges of storage elements with a low capacity, while on the other hand, during charging processes, lead to excess charging of storage elements which are fully charged prematurely. This behaviour is generally divergent; in other words, even small differences between the individual storage elements lead during the course of time to the developments described above when only a sufficiently large number of charging/discharging cycles are executed. The developments described above initially lead to damage or failure of the affected storage element and can finally, in a type of chain reaction, cause the premature failure of the entire energy storage unit.
In order to avoid effects of this nature (which are in practise unavoidable) which are caused by differences between individual storage elements, several authors have already suggested different methods with which a symmeterisation of the charge state of the individual storage elements is produced, e.g. Schmidt et al. in the aforementioned conference paper, and in EP 0 432 639 A2, N. Kutkut et al. in “Dynamic equalization techniques for series battery stacks”, Telecommunications Energy Conference 1996 (Intelec), Boston, 6 to 10 Oct. 1996, IEEE 0-7803-3507-4/96, p. 514-521, and Ridder in EP 1 283 580 A2. These suggestions are all based on the idea that the voltage of the storage elements should be monitored, and that charge should be removed from storage elements with a higher charge (wherein with some suggestions, charge is removed from all storage elements, with more charge being removed from storage elements with a higher charge than from those with a low charge). While with earlier suggestions (which are described for example by Schmidt in the aforementioned conference paper) the energy removed from the more highly charged storage elements was dissipated in heat resistances, according to more recent suggestions, the removed charge is fed back to the energy storage unit (i.e. in effect to the other storage elements). A re-storage of this nature is more effective, since with this method, the only energy loss is that which arises as a result of the re-storage process, rather than the entire quantity of re-stored energy. With these more recent suggestions, the working life of energy storage units of the type named above can be considerably extended, with a relatively high degree of effectiveness.
The symmetry connection described by Ridder in EP 1 283 580 A2 is constructed of charge removal connections which are assigned to the individual storage elements, and which operate essentially autonomously. An upstream control unit determines the voltage thresholds for the charge removal connections (EP 1 283 580 A2, paragraph [0027]).